Distributed Generation (DG) involves generating electricity from many small energy sources. For example, solar panels on a building, wind turbines in an open field or hydro-engines in an ocean can act as small energy generation sources, or DG generators, that provide energy to a utility grid. Utility consumers, such as buildings located in urban and other densely populated locations, connect DG generators to the utility grid in order to supply power back to the grid to offset some or all use of power supplied by the utility grid. For example, the consumer may trade energy produced by their DG generator to the utility (such as via a metering agreement) in order to lower their overall energy costs.
At times, a power failure of the utility grid or other service interruption will occur that causes the consumer to be disconnected from and not receive power from the utility grid. However, the consumer will continue to receive power from the DG generator. Due to the service interruption, the DG generator will be disconnected from the utility grid, yet will continue to energize the portion of the utility grid allotted to the consumer. This condition, known as “islanding,” can be problematic. Islanding occurs when a portion of the utility grid is becomes isolated from the rest of the grid yet continues to receive power from a DG generator. Some hazards associated with islanding include:
(1) Utility workers sent out to repair the utility grid may not be aware that the portion of the utility grid is receiving power from a DG generator even though the utility grid itself is not powered. Serious injury or death may occur should a worker make contact with a portion of the utility grid;
(2) The utility has little or no control over the voltage or frequency supplied to an islanded location which creates the possibility of damage or potential hazards at the islanded location;
(3) It may interfere with restoration of normal service; and so on.
To that end, there have been many systems developed that attempt to quickly and effectively detect islanding. These systems aim to detect islanding within a certain time period and aim to minimize the range of the Non-Detection Zone (NDZ). The NDZ is the range of the local loads (i.e., the load on the islanded location containing the DG generator) that are not detected by an anti-islanding system.
Some of these systems employ active detection methods within inverters associated with DG generators within the utility grid. An active system will deliberately introduce a change and/or disturbance (such as a transient or harmonic disturbance) through connections to the utility grid and monitor any response back from the utility grid. When the utility grid is connected and supplying power to the location containing the DG generator, there will be little or no effect to the frequency, voltage, or impedance of the utility grid. However, when the utility grid is disconnected from the DG location (or otherwise not supplying power), the disturbance will affect the load on the utility grid, and the inverter will cease converting and/or delivering power to the DG location within the utility grid. Thus, anti-islanding will take place.
For inverters containing microcontrollers, such as many photovoltaic (PV) inverters, a number of positive-feedback mechanisms have been developed that can be implemented into the inverter to detect islanding. For example, the “Frequency Bias” Method injects a waveform into the utility grid that attempts to change the frequency of the utility grid. The utility grid will absorb any attempted changes to the frequency when the utility grid is connected to the DG location. However, when the grid is disconnected, the waveform will reflect the changed frequency. This can lead to a frequency and phase error at subsequent measurement cycles. Once the increase (or, decrease) in frequency of the inverter reaches a threshold value, the system identifies an islanding condition and the causes the inverter to stop converting power.
The Frequency Bias method must introduce a large frequency bias to be a reliable anti-islanding mechanism, which can lead to the generation of unwanted audible noise and harmonic currents that deteriorate the power factor of an inverter. Additionally, the method is not preferable for utility grids containing multiple inverters unless every inverter contributes a frequency bias having the same direction so as not to cancel out one another. This can only be achieved when every inverter manufacturer agrees on a direction, which is not likely. Additionally, the Non-Detection Zone (NDZ) of the Frequency Bias Method is generally larger than other active methods, making it a relatively poor method for detection.
The “Sandia Frequency Shift” (SFS) Method extends the Frequency Bias method by injecting a waveform into the utility grid that attempts to change the frequency of the voltage of the utility grid. When the utility grid is disconnected, the measured frequency increases (or, decreases), and a difference in frequency (Δf), or error frequency, between the measured frequency and an average value of the frequency also increases. The inverter will likewise increase its frequency to compensate for the increased frequency error. The system will shut down the inverter once the inverter frequency reaches a threshold value above or below a normal operational value.
Despite having a relatively small NDZ for active systems, the SFS method also suffers from a number of drawbacks. For example, the instability of the power output can lead to undesirable transient behaviors in weak utility grids. This can be somewhat corrected by reducing loop gain, but gain reduction in this system increases the time to reach the threshold frequency error. Additionally, this method can not accurately detect islanding where there is no detectable frequency error (such as when generated power and load power are balanced).
These and other problems exist with respect to current anti-islanding systems and methods implemented within distributed generation inverters.